Method and apparatus for recording three-dimensional distribution of light backscattering potential in transparent and semi-transparent structures

ABSTRACT

An apparatus is disclosed for generating data representative of a three-dimensional distribution of the light backscattering potential of a transparent or semi-transparent object such as a human eye. The apparatus includes an interferometer, both the reference beam and measurement beam of which are directed toward the object and reflected by respective reference and measurement sites thereof, such that axial motion of the object during measurement affects both beams equally. The measurement beam is raster scanned transversely across each measurement site for which data is obtained. Also, the frequency of one of the beams is shifted by a non-moving frequency shifter, such that the reflected beams combine and are modulated by a heterodyne beat frequency, which is detected when the object path difference is matched with the interferometer path difference. Because the non-moving frequency shifter can effectively generate a beat frequency of about 40 MHz, relatively rapid transverse and longitudinal scanning are facilitated.

FIELD OF THE INVENTION

1. The present invention relates generally to determining lightbackscattering at various depths in transparent and semi-transparentstructures, and more particularly to methods and apparatus fordiagnosing human eye conditions by detecting the reflection of lightfrom various layers in the eye.

BACKGROUND

2. Detailed structural knowledge of transparent and semi-transparentobjects can be gained by measuring the reflective backscattering oflight from various layers in the objects. Such structural knowledge canbe useful in various industrial applications such as semiconductor chipfabrication. Furthermore, knowing the precise structure of the eye of ahuman patient is useful in diagnosing certain conditions of the eye,including, for example, glaucoma.

3. German Patent No. 3201801A1 to Fercher discloses a method referred toas partial coherence interferometry (“PCI”) in which light having ashort coherence length is combined with a Michelson interferometer tolocate the positions of reflecting surfaces within an object. In PCI, ameasurement light beam from the interferometer is directed against aparticular reflecting surface in the object, and a reference light beamis directed against a known reference surface. The position of thereflecting surface in the object is determined by matching the length ofthe known reference path that is traversed by the reference light beamto the unknown object path length that is traversed by the measurementlight beam.

4. The above-discussed PCI method results in the generation of so-calledoptical A-scans, which can be thought of as plots of backscattered lightintensity as a function of depth within the object. U.S. Pat. No.5,321,501, incorporated herein by reference, discloses a techniquereferred to as optical coherence topography (“OCT”) in which severalA-scans are combined to effectively map the depth of an object.

5. The invention disclosed in the '501 patent is embodied in thecommercial OCT instrument sold by Humphrey Instruments/Carl Zeiss.Unfortunately, the Humphrey-Zeiss instrument permits measuring onlylongitudinal sections, not transverse sections, and furthermore itrequires that the section geometry be defined a priori. The presentinvention understands that it is desirable to measure both longitudinaland transverse sections to thereby generate a three dimensional map ofthe object, and that it is also desirable that the section geometry notbe defined a priori. Moreover, the present invention recognizes thatbecause the reference surface of the Humphrey-Zeiss instrument is notpart of the object to be measured, the precision of the instrument canbe degraded by axial movement of the object during measurement.

6. In addition to the above-mentioned drawbacks, the present inventionrecognizes that the speed of measurement of prior instruments isrelatively slow, and that prolonged measurement time has undesirableconsequences, as set forth in the following discussion. As mentionedabove, the object path length is matched with the reference path lengthin PCI applications, including OCT. This matching, when it occurs, isindicated by the presence of interference fringes caused by theinterference of the return reference beam with the return measurementbeam. In early PCI applications, the interference fringes were visuallydetected, which significantly lengthened the time required to gather thebackscattering data at the various layer depths. Unfortunately,prolonged data gathering periods limits the resolution of the data whenthe object being analyzed moves. In the case of the human eye,microsaccidic eye movements tend to limit the resolution of the data.

7. Accordingly, to facilitate more rapid detection of interferencefringes, Hitzenberger et al., in an article entitled “Eye LengthMeasurement by Laser Doppler Interferometry (“LDI”)”, Int'l Conf. onOptics within Life Sciences, Garmisch-Partenkirchen, 1990, proposedetecting the fringes by heterodyning. Specifically, the Hitzenberger etal. article discloses moving a reference mirror at constant speed tocause a Doppler frequency shift in one of the beams, causing thegeneration of a detectable “beat” frequency when the reference beam andmeasurement beam interfere with each other as they return from theobject being measured.

8. As recognized by the present invention, however, while Doppler-basedheterodyne detection is all improvement over the visual detectionmethod, the use of mechanically moving parts nevertheless limits thespeed of measurement by limiting the magnitude of the induced frequencyshift, which is proportional to the speed of the reference mirror.Additionally, the present invention recognizes that mechanically-basedheterodyning techniques can induce a varying beat frequency, causingdemodulation complications, and also requiring a relatively large filterbandwidth (used during demodulation) to account for the variations. Thelarge filter bandwidth in turn reduces the signal to noise ratio of theinstrument.

9. Accordingly, it is an object of the present invention to provide amethod and apparatus for generating a map of a transparent orsemi-transparent object. Another object of the present invention is toprovide a method and apparatus for rapidly generating a map of a humaneye. Still another object of the present invention is to provide amethod and apparatus for rapidly generating a map of a human eye that iseasy to use and cost-effective, and that is not degraded by axial motionof the eye during measurement.

SUMMARY OF THE INVENTION

10. An apparatus is disclosed for detecting the distribution of lightbackscattering potential in an object, such as a human eye. Theapparatus includes an interferometer which emits plural preferably shortcoherence-length light beams, with at least one of the light beams beingdirectable toward the object. The light beams are reflected byrespective surfaces to establish respective reflected light beams. Atleast one non-moving frequency shifter is positioned in at least onepath of the light beams, and a receiver receives the reflected beams andgenerates a signal representative thereof. As intended by the presentinvention, the signal is usable for determining light backscatteringsites in the object.

11. In a preferred embodiment, the plural light beams include at least areference beam directed at a reference surface defined by the object anda measurement beam directed at a measurement site defined by the object.With this structure, axial motion of the object relative theinterferometer equally affects the reference beam and measurement beam.

12. As disclosed in detail below, the frequency shifter changes thefrequency of light beams passing therethrough. The frequency shifter isselected from the group of frequency shifters including acousto-opticfrequency shifters, and phase modulators.

13. Preferably, the interferometer defines a reference arm and ameasurement arm, and a first difference is established between thelengths of the arms. The reference beam travels a reference distancerelative to the object, the measurement beam travels a measurementdistance relative to the object, and a second difference is establishedbetween the reference distance and measurement distance. At least onearm length can then be established such that the first difference bearsa proportional relationship to the second difference, and a beatfrequency is received by the receiver when the differences aresubstantially matched. In a particularly preferred embodiment theinterferometer includes at least one translationally movable path delayunit to selectively establish the at least one arm length.

14. If desired, polarized light can be used. When polarized light isused, a polarizer can be positioned in the light entrance path of theinterferometer, and a half wave plate can be positioned in the arm ofthe interferometer in which the frequency shifter is positioned. Also, aquarter wave plate can be positioned in the light exit path.

15. In addition to the above structure, at least one spatial filter canbe disposed in at least one of the arms to improve the quality of a wavefront of light passing therethrough. Still further, a scanning mirror ispositioned in the measurement arm and is tiltable in at least one degreeof freedom to selectively establish a direction of propagation of themeasurement beam. A coherence layer correcting element is juxtaposedwith the scanning mirror to alter a length of a path traversed by themeasurement beam in proportion to an angle established between themeasurement beam and its null direction, typically the direction of thereference beam.

16. In another aspect, an apparatus for matching an object pathdifference with an interferometer path difference and identifying thematching by heterodyne detection of a reference beam and a measurementbeam includes means for shifting the frequency of at least one of: thereference beam, and the measurement beam, by at least ten megaHertz.Thereby, a beat frequency is generated when the object path differencebears a predetermined relationship to an interferometer path difference.

17. In still another aspect, a method is disclosed for generating datarepresentative of a three-dimensional distribution of the lightbackscattering potential of an object defining at least one referencesurface and plural measurement sites. The method includes directing areference beam from a reference arm of an interferometer against thereference surface, and also directing a measurement beam from ameasurement arm of the interferometer against the measurement site. Themethod further includes shifting the frequency of at least one beam byat least ten megahertz. Reflections of the beams from the surfaces arecombined and detected.

18. The details of the present invention, both as to its structure andoperation, can best be understood in reference to the accompanyingdrawings, in which like reference numerals refer to like parts, and inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

19.FIG. 1 is a schematic diagram of a first embodiment of the presentinvention using a Michelson interferometer, showing the measurementmirror in a null position in phantom and showing the measurement mirrorin a measurement position in solid lines;

20.FIG. 2 is a schematic diagram of a second embodiment of the presentinvention using a modified Mach-Zehnder interferometer;

21.FIG. 3 is a schematic diagram of an alternate embodiment of a portionof the apparatus shown in FIG. 2, showing a coherence layer correctingelement and showing a coherence layer in solid lines as it would appearwithout the correcting element, and showing the coherence layer indashed lines as it would appear with the correcting element;

22.FIG. 4 is a block diagram of the electrical control components; and

23.FIG. 5 is a flow chart of the process used by the apparatus togenerate a three-dimensional map of an object.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

24. Referring initially to FIG. 1, an apparatus is shown, generallydesignated 10, for generating data representative of thethree-dimensional distribution of light backscattering sites, and theirrespective reflectivities, in a transparent or semi-transparent object12, such as a human eye. That is, the apparatus 10 measures thereflection of light by various sites within the object 12 along thelongitudinal axis, i.e., z-axis, with the understanding that the z-axisis orthogonal to the transverse axes, i.e., the x and y axes. It is tobe understood, however, that the apparatus 10 can be used to generatedata representative of the three-dimensional distribution of lightbackscattering potential in objects other than eyes.

25.FIG. 1 shows a simplified embodiment that uses a Michelsoninterferometer 14. As shown, a short coherence, broadband light sourceLS directs a beam of light into an entrance path 16 of theinterferometer 14. In the preferred embodiment, the light source LSincludes a superluminescent diode and collimating optics. Lightpropagating along the entrance path 16 impinges upon an entrancebeamsplitter BS, which splits the beam into a reference beam propagatingalong a reference arm or path 18 and a measurement beam propagatingalong a measurement arm or path 20. The reference beam is reflected by areference mirror M back to the entrance beamsplitter BS, through whichit propagates along a dual beam path DB to an exit beamsplitter BS′.Similarly, the measurement beam is reflected by a measurement mirror M′back to the entrance beamsplitter BS, which reflects the measurementbeam to propagate along the dual beam path DB to the exit beamsplitterBS′ such that the reference beam and the measurement beam form a coaxialdual beam.

26. To vary the length of the optical path through which at least themeasurement beam propagates through the interferometer 14, at least themeasurement mirror M′ is translationally movable between a nullposition, indicated by dashed lines in FIG. 1, and plural measurementpositions (only a single measurement position shown and indicated insolid lines in FIG. 1). When the measurement mirror M′ is in the nullposition, the distance traversed by the measurement beam through theinterferometer 14 is equal to the distance traversed by the referencebeam through the interferometer 14. On the other hand, when themeasurement mirror M′ is in a measurement position that is distancedfrom the null position by an interferometer differential distance of“d”, the distance traversed by the measurement beam through theinterferometer 14 is equal to the distance traversed by the referencebeam through the interferometer 14, plus two times “d”. In this lattercase, an interferometer optical path difference of magnitude “2d” isestablished between the distances traversed by the reference andmeasurement beams through the interferometer 14. It is to be understoodthat the measurement mirror M′ need not actually be moved to the nullposition, once it is known, but rather need only be movable to thevarious measurement positions.

27. In accordance with the present invention, at least one frequencyshifter FS is positioned in at least one of the arms 18, 20. In theembodiment shown, the frequency shifter FS is positioned in thereference arm 18, but it is to be understood that the frequency shifterFS can be positioned in the measurement arm 20 or that a secondfrequency shifter (not shown) can be positioned in the measurement arm20.

28. As intended by the present invention, the frequency shifter FSshifts the frequency of light propagating through it by an amount Δf tofacilitate heterodyne detection of interference fringes caused by theinterference of the reference beam with the measurement beam after thebeams have been reflected by the object 12. The detection of suchfringes indicates that the interferometer optical path difference,discussed above, matches the object optical path difference between thereference and measurement beams within the object 12, discussed morefully below. By “match” is meant that the two optical path differencesbear a predetermined proportional relationship to each other, e.g., oneto one. Preferably, the precision of the match is on the order of thecoherence length of the light source.

29. In a preferred embodiment, the frequency shifter FS is non-moving.By “non-moving” is meant functionally non-moving, i.e., that thefrequency shifter FS preferably does not have to be moved to shift thefrequency of light. Thus, the frequency shifter FS can be establishedby, e.g, an acousto-optic frequency shifter or a phase modulator. It isto be understood, however, that the frequency shifter FS can beestablished by a moving mirror, a moving cube, a moving glass plate, amoving beam splitter, a moving retroreflector, or a moving lens,provided that the frequency shifter FS can be moved sufficiently quicklyto generate a heterodyne beat frequency of at least ten million Hertz(10 MHz), and more preferably a heterodyne beat frequency of at leastforty million Hertz (40 MHz).

30. As shown in FIG. 1, the beams from the interferometer 14 arereflected by the exit beam splitter BS′ out of the interferometer 14 andalong an object path 22, toward the object 12. Both beams ire reflectedby a strongly reflecting reference surface of the object 12, preferablythe anterior surface 24. However, if the anterior surface 24 isinsufficiently reflective, a glass plate (not shown) or other highlyreflective yet transmissive object can be positioned in front of theobject 12 at a fixed distance to it to function as the referencesurface.

31. Additionally, both beams are reflected by measurement sites alongthe z-axis within the object 12. FIG. 1 shows one such measurement site26 that is distanced from the reference surface 24 by an objectdifferential distance “z”. It is the purpose of the present invention tomeasure the various object differential distances “z” from the referencesurface 24 to the various internally reflective measurement sites 26 ofthe object 12. Moreover, the present invention not only measures thedistances from the reference surface 24 to the various measurement sites26, but also measures the backscattering potential (i.e., reflectivity)of the various measurement sites 26. All of the reflected beamspropagate back through the exit beamsplitter BS′ and can be focussed ona receiver, such as a photodetector PD, by a lens L1.

32. In accordance with present principles, when the interferometerdifferential distance “d” equals one of the object differentialdistances “z”, the corresponding reflected reference beam from thereference surface 24 and reflected measurement beam from the measurementsite 26 will interfere. Owing to the fact that the frequency shifter FSshifted the frequency of one of the beams by an amount Δf, when thebeams interfere they generate an optical so-called beat signalcharacterized by intensity modulations with frequency Δf, which acts asa carrier frequency.

33. The skilled artisan will appreciate that the beat signal isconverted to an electrical signal by the photodetector PD. The signal isthen sent to a signal processor SP for demodulation. The demodulationcan be undertaken by band pass filtering the signal with a filtercentered on Δf and detecting its envelope, the magnitude of which isrelated to the backscattering potential of the measurement site 26 ofinterest. Then, the signal is converted to digital format by an analogto digital converter AD, and then stored in a computer PC forcorrelating the processed signal to a measurement site having depth “z”relative to the reference surface 24 and a backscattering potential asrepresented by the contrast of the interference fringes, usingcorrelation principles understood in the art. Alternatively, the beatsignal can be immediately converted to a digital signal and thenprocessed numerically by a computer or digital signal processor.

34. Thus, it may now be appreciated that the measurement mirror M′ ispositioned at various locations to establish various values for theinterferometer differential distance “d”, with the interference fringesbeing recorded (or not) at each position as indicative of whether andhow strong a backscattering site exists at a distance “z” from thereference surface 24 in the object 12. Since both the reference beam andmeasurement beam are reflected by the object 12, the apparatus 10 isinsensitive to axial motion of the object 12 during measurement.

35. The backscattering sites at a distance “z” from the referencesurface 24 are all in a common “coherence layer” at distance “z”. By“coherence layer” is meant that layer within the object sought to bemapped whose distance to the reference point (the point on the object atwhich the reference beam is reflected) is equal to the interferometerpath length difference. Any backscattering structure within thecoherence layer will reflect the measurement beam such that itinterferes with the reference beam. As understood by the presentinvention, the position of the coherence layer is determined by thevalue of the interferometer path difference, and the curvature of thecoherence layer is determined by the geometry of the light path.

36. As described more fully below in reference to FIG. 2, to record thetwo dimensional backscattering potential distribution within eachcoherence layer, either the object 12 or the measurement beam can bemoved in a raster scan for each interferometer differential distance“d”. To avoid aliasing artifacts, the beat frequency Δf should be largerthan the frequencies caused by intensity variations during the rasterscanning, which can be caused by locally varying areas within thecoherence layer. It may now be further understood that the presentinvention preferably uses a non-moving frequency shifter FS, becausesuch a frequency shifter is better able to produce relatively large beatfrequencies Δf than are moving Doppler-type devices.

37. Now referring to FIG. 2, a particularly preferred embodiment of thepresent apparatus is shown, generally designated 100, for generatingdata representative of the three-dimensional distribution of lightbackscattering sites and their reflectivity in a transparent orsemi-transparent object 102, Such as a human eye. It is to be understoodthat the apparatus 100 shown in FIG. 2 is identical in operation andpurpose to the apparatus 10 shown in FIG. 1, with the exceptions notedbelow. Instead of a Michelson interferometer, the apparatus 100 uses aMach-Zehnder interferometer 104, which establishes a base on which thebelow-disclosed components are movably or fixedly mounted as set forthherein. As recognized by the present invention, the use of such aninterferometer facilitates processing the two beam componentsindependently of each other such that each component can be focussedindependently of the other at its own respective depth within the object102, and such that parallel wave fronts of the return beams can beobtained at the photodetector.

38. As shown in FIG. 2, the apparatus 100 includes a short coherence,broadband light source LS that directs a beam of light into an entrancepath 106 of the interferometer 104. To avoid excessive frequency shiftsand/or to avoid the cancellation of frequency shifts, as well as tominimize degradation in signal-to-noise ratio that could otherwise occurif the light beam passed through the frequency shifter FS twice, insteadof once, a polarizer P is positioned in the entrance path 106 or otherappropriate location between the light source LS and frequency shifter.

39. Light propagating along the entrance path 106 impinges upon anentrance beamsplitter BS 1, which splits the beam into a reference beampropagating along a reference arm or path 108 and a measurement beampropagating along a measurement arm or path 110.

40. First considering the reference arm of the interferometer 104, lightpropagating along the reference arm 108 is reflected by a fixedreference mirror M 1 toward a first polarizing beamsplitter PBS 1. It isto be understood that the polarizing beamsplitters discussed herein areused only when polarized light is used, so that the reference beamtraverses the frequency shifter only once; otherwise, the polarizingbeamsplitters can be replaced with respective mirrors. In the lattercase, the frequency shifter is positioned anywhere in the referencebeam, or indeed any where in the measurement beam.

41. The first polarizing beamsplitter PBS 1 permits light from thereference mirror M 1 to pass therethrough. Owing to the polarization ofthe light effected by the polarizer P, substantially none of thereference beam from the reference mirror M 1 is reflected by the firstpolarizing beamsplitter PBS1.

42. The reference beam continues from the beamsplitter PBS 1 to at leastone preferably non-moving frequency shifter FS, where its frequency isshifted in accordance with principles discussed previously. Then, thereference beam may pass through an optionally rotatable half wave plateIIWP, which rotates the polarization of the reference beam to itsoriginal orientation or other desired orientation. The half wave plateHWP is necessary only if the frequency shifter FS rotates thepolarization plane of the frequency shifted beam, such as might occurfor certain types of frequency shifters, e.g., AO frequency shifters. Asintended by the present invention, once the half wave plate HWP isoriented in the appropriate position it is not necessary to furtherrotate the half wave plate HWP.

43. From the half wave plate HWP, the reference beam propagates througha second polarizing beamiisplitter PBS 2, it being understood that thesecond polarizing beamsplitter PBS 2 is configured for permittingsubstantially all light having the polarization of the reference beam topass therethrough, without reflecting the light. As stated above, whenthe frequency shifter is not bypassed in the return direction, thesecond polarizing beamsplitter PBS 2 can be replaced by a mirror.

44. To prevent light beams that travel back through the contralaterallight path of the interferometer 104 from affecting the data, and toimprove the quality of the wave fronts of the reference beams returningfrom the object 102, a reference spatial filter S 1 is positioned in thereference arm 108. In one preferred embodiment, the reference spatialfilter S 1 includes a first reference lens L 5 that focusses light ontoan aperture 112 in a reference aperture plate A 1, and a secondreference lens L 6 that receives light from the aperture plate and inturn focusses the light onto the reference surface of the object 102(the vertex of the cornea when the object 102 is a human eye). In theembodiment shown, in which the object 102 is a human eye, the referencesurface of the object 102 is an anterior surface 114 of the object 102.As can be appreciated in reference to FIG. 2, reference beams returningfrom the object 102 are focussed by the second reference lens L 6 ontothe aperture 112, and when the return reference beam is diffractedthrough the aperture 112, it passes through the first reference lens L5, which refracts the beam to have a plane parallel wave front.

45. From the reference spatial filter S 1, the reference beam propagatesto an exit beamiisplitter BS 2, where it is transmissively passed out ofthe interferometer 104 to an object path 116. When the frequency shifterFS is to be bypassed by returning light beams, a quarter wave plate QWPis positioned in the object path 116 as shown.

46. From the quarter wave plate QWP, the reference beam propagates tothe object 102, where at least a portion of the reference beam isreflected by the reference surface 114 of the object 102 and back alongthe path just described. Specifically, the reflected reference beampasses through the quarter wave plate QWP, exit beamsplitter BS 2, andback into the reference arm or path 108 of the interferometer 104. Asdiscussed above, the return reference beam is conditioned by thereference spatial filter S 1, and then the return reference beampropagates to the second polarizing beamsplitter PBS 2.

47. Owing to the polarization of the return beam, the return beam issubstantially completely reflected by the second polarizing beamsplitterPBS 2 to a reference path delay unit PDU 1. As shown, the reference pathdelay unit PDU 1 includes first and second reference delay mirrors M 3,M 2, with the return beam being reflected by the first reference delaymirror M 3 to the second reference delay mirror M 2. In turn, the secondreference delay mirror M 2 reflects the beam to the first polarizingbeamsplitter PBS1, and then back along the remainder of the referencearm 108 to the entrance beamsplitter BS 1. It may now be appreciatedthat owing to above-described cooperation of structure, the referencebeam passes through the non-moving frequency shifter FS only once.

48. In accordance with the present invention, the reference path delayunit PDU 1 is translationally movable in the directions indicated by thearrows 118, to establish a reference path length differential distanced, as shown. Suitable means, such as a small servo, can be used to movethe reference path delay unit PDU 1.

49. From the entrance beamsplitter BS 1, the light passes through anexit lens L 1, which focusses the light onto a photodetector PD forsubsequent processing by a signal processor SP, analog to digitalconverter AD, and computer PC as described above in reference to FIG. 1.Thus, beam components exiting the entrance beamsplitter BS 1 haveparallel wave fronts and can be focussed at the photodetector PD, inwhich case the beams are confocal to their respective conjugate pointsat or within the object 102. In contrast, beam components propagatingback through the contralateral interferometer path will not haveparallel wave fronts when they exit the entrance beamsplitter BS 1 andconsequently will not pass the confocal aperture that is effectively infront of the photodetector PD. Accordingly, beam components propagatingback through the contralateral interferometer path will not be used forthe interferometric distance measurements.

50. Now considering the measurement arm 110, light propagating along thearm 110 from the entrance beamsplitter BS 1 is reflected by a firstfixed measurement path mirror M 10 to a measurement path delay unit PDU2. As shown, the measurement path delay unit PDU 2 includes first andsecond measurement delay mirrors M 9, M 8, with the beam being reflectedby the first measurement delay mirror M 9 to the second measurementdelay mirror M 8. In turn, the second measurement delay mirror M 8reflects the beam to a second fixed measurement path mirror M 7. It isto be understood that designations M 8, M 9 can alternatively bereplaced by a retroreflector that can be used in lieu of first andsecond measurement delay mirrors.

51. In accordance with the present invention, the measurement path delayunit PDU 2 can be translationally movable in the directions indicated bythe arrows 120, to establish a measurement path length differentialdistance d₂ as shown. By providing two path delay units PDU 1, PDU 2,the detection of measurement sites that are close to the referencesurface 114 of the object 102 (i.e., that are shallow) is facilitated.Suitable means, such as a small servo, can be used to move themeasurement path delay unit PDU 2.

52.FIG. 2 shows that the light from the second measurement path mirror M7 propagates to a dispersion compensation element DC. Per the presentinvention, the dispersion compensation element DC is configured tocompensate for the net difference of the group dispersive effects of theoptical elements in the two arms 108, 110, and those caused by thedifferent path lengths within the object 102. In so doing, thedispersion compensation element DC inmproves the axial resolution of theapparatus 100. In one preferred embodiment, the dispersion compensationelement DC is established by two wedge-shaped glass plates juxtaposed asshown, with at least one of the wedges being movable in the directionsindicated by the arrows 6 to establish the distance through which thelight must pass through the wedges to thereby compensate for dispersiveeffects.

53. While FIG. 2 shows the dispersion compensation element DC positionedin the measurement arm 110, when the group dispersive effect caused bydifferent path lengths within the object 102 is larger than that causedby the additional optical elements in the reference arm 108, thedispersion compensation element DC is positioned in the reference arm108. Or, a respective dispersion compensation element can be positionedin each arm 108, 110 of the interferometer 104.

54. A measurement spatial filter S 2 is positioned in the measurementarm 110. The measurement spatial filter S 2 is in all essential respectsidentical in construction and operation to the reference spatial filterS 1 discussed above. Accordingly, the measurement spatial filter S 2includes a first measurement lens L 2 that focusses light onto anaperture 122 in a measurement aperture plate A 2, and a secondmeasurement lens L 3 that receives light from the aperture plate and inturn directs the light onto a scanning mirror M 4.

55. As intended by the present invention, the scanning mirror M 4 scansthe measurement beam across the particular coherence layer sought to bemapped in the object 102. In the preferred embodiment, the mirror M 4 istiltably moved about both the x-axis and y-axis in a raster scan, asindicated by the x arrows and y arrows, which are shown orthogonal toeach other in FIG. 2. Accordingly, the scanning mirror M 4 is movablymounted on the interferometer 104 by suitable means, e.g., a gimbal, forpermitting tiltable motion of the mirror M 4 in two degrees of freedom.Alternatively, two mirrors can be used in lieu of the scanning mirror M4, each being tiltable in a single degree of freedom. One or more servoscan be used to move the scanning mirror M 4.

56. As shown in FIG. 2, the scanning mirror M 4 reflects the measurementbeam to a measurement exit lens L 4. The measurement beam propagatesthrough the lens L 4 to the exit beamsplitter BS 2, and thence onto theobject 102. As indicated in FIG. 2, the measurement beam is reflected bya measurement site in the object 102 (e.g., the retina or deeper layersof the ocular tundus when the object 102 is a human eye), and returnsvia the measurement path or arm 110 just described to the photodetectorPD.

57. As intended by the present invention, the measurement path lenses L3 and L 4, in cooperation with the refractive elements of the object102, focus the measurement beam onto the desired measurement site.Refractive errors of the object 102, e.g., a human eye, can beaccommodated for using additional measurement path lenses (not shown),or with movable lenses or lenses having variable focal lengths. In anycase, the focal length of the measurement exit lens L 4 is establishedto image the pivot point of the scanning mirror M 4 substantially ontothe nodal point of the object 102. Moreover, either or both of themeasurement path lenses L 3, L 4 can be translationally moved, e.g., byservos, or have their focal lengths changed, in consonance with varyingthe interferometer path delay, to focus the beam onto the measurementsite sought to be detected.

58. With further regard to the path delay within the interferometer 104,it may now be understood that when the frequency shifter FS is bypassedin the return direction (i.e., that when the polarizing beamsplittersPBS 1 and PBS 2 are used), the interferometer path difference 2d₁−4d₂(plus differences that are attributable to different optical elements inone path 108, 110 vis-a-vis the other path 110, 108.) Otherwise, (i.e.,when the frequency shifter FS is not bypassed in the return direction),the interferometer path difference=4d₁−4d₂ (plus differences that areattributable to different optical elements in one path 108, 110vis-a-vis the other path 110, 108.)

59.FIG. 3 shows an additional detail of the interferometer 104. Becausethe transversal distribution of backscattering sites is determined bymoving the scanning mirror M 4 to tilt the measurement beam as describedabove, the coherence layer within the object will be curved. In the caseof the human eye, the curvature of the coherence layer can be differentfrom the curvature of the surfaces (e.g., the retina) to be measured, asshown in solid lines in FIG. 3. The present invention recognizes thatthe light path of the measurement beam be established such that thecurvature of the coherence layer is about equal to the curvature ofstructures, e.g., the retina, sought to be imaged.

60. To accommodate such differences in curvature, a coherence layercorrecting element CLC can be positioned between the scanning mirror M 4and the lens L 4 to alter the optical path length of the tiltedmeasurement beam as a function of the scanning angle, therebyeffectively altering the curvature of the coherence layer to match thatof the surfaces to be measured as shown in dashed lines in FIG. 3. Inthe embodiment shown in FIG. 3, the coherence layer correcting elementCLC is a glass plate. Alternatively, the coherence layer correctingelement can be two glass wedges similar to the dispersion compensationelement DC, with the understanding that the wedges can be moved (e.g.,by servos or other means) to establish their relative transversalposition as appropriate for the scan angle and for the curvature of themeasured surface.

61.FIG. 4 shows that the computer PC can control various componentswithin the interferometer 104 in accordance with the principlesdiscussed above. More particularly, the computer PC can send signals toa digital to analog converter (DAC) 124 to control one or more servos126 for moving the scanning mirror M 4, in accordance with thedisclosure above. Also, the computer PC can send signals to a digital toanalog converter (DAC) 128 to control one or more servos 130 fortranslationally moving the reference path delay unit PDU 1, inaccordance with the disclosure above. Moreover, the computer PC can sendsignals to a digital to analog converter (DAC) 132 to control one ormore servos 134 for translationally moving the measurement path delayunit PDU 2, in accordance with the disclosure above. Furthermore, thecomputer PC can send signals to a digital to analog converter (DAC) 136to control one or more servos 138 or moving one or more of the lensesand/or coherence layer correcting element CLC and/or dispersioncompensating element in accordance with the disclosure above.

62.FIG. 5 illustrates the logic undertaken by the computer PC ingenerating data that is useful for rendering a three-dimensional map ofa transparent or semi-transparent object. As set forth below, togenerate a three-dimensional map, consecutive coherence layers aresequentially raster scanned, with the distance between coherence layersbeing about equal to the coherence length of the light source.

63. Accordingly, commencing at block 140, a start and an end value, aswell as an incremental step length of the interferometer path lengthdifferential, are defined. Beginning with the start value of the pathlength differential, the logic undertakes the following steps. Thescanning mirror M 4 is controlled, using the scanning mirror servo 126,to move the measurement beam in a raster scan as disclosed. At block144, at each point in the raster scan the contrast of the interferencefringes for the present value of the path length differential isrecorded. These values can be correlated to backscattering potential andcoherence layer depth, relative to the reference layer of the object, asdiscussed above. When the raster scan for a coherence layer has beencompleted, the logic moves to block 146 to increment the interferometerpath length differential by the step length by signalling one or both ofthe PDU servos 130, 134 to move. The logic then loops back to block 142to raster scan the new coherence layer. These steps are repeated untilthe end value of the interferometer path length differential is reached.

64. While the particular METHOD AND APPARATUS FOR RECORDINGTHREE-DIMENSIONAL DISTRIBUTION OF LIGHT BACKSCATTERING POTENTIAL INTRANSPARENT AND SEMI-TRANSPARENT STRUCTURES as herein shown anddescribed in detail is fully capable of attaining the above-describedobjects of the invention, it is to be understood that it is thepresently preferred embodiment of the present invention and is thusrepresentative of the subject matter which is broadly contemplated bythe present invention, that the scope of the present invention fullyencompasses other embodiments which may become obvious to those skilledin the art, and that the scope of the present invention is accordinglyto be limited by nothing other than the appended claims, in whichreference to an element in the singular is not intended to mean “one andonly one” unless explicitly so stated, but rather “one or more”.

What is claimed is:
 1. An apparatus for detecting the distribution of light backscattering potential in an object, comprising: an interferometer emitting plural light beams, at least one of the light beams being directable toward the object, the light beams being reflectable by respective surfaces to establish respective reflected light beams; at least one non-moving frequency shifter positioned in at least one path of the light beams; and a receiver receiving the reflected beams and generating a signal representative thereof, the signal being usable for determining the distribution of light backscattering potential in the object.
 2. The apparatus of claim 1 , wherein the plural light beams include at least a reference beam directed at a reference surface defined by the object and a measurement beam directed at a measurement site defined by the object, such that axial motion of the object relative the interferometer equally affects the reference beam and measurement beam.
 3. The apparatus of claim 1 , wherein the frequency shifter changes the frequency of light beams passing therethrough.
 4. The apparatus of claim 3 , wherein the frequency shifter is selected from the group of frequency shifters including acousto-optic frequency shifters, and phase modulators.
 5. The apparatus of claim 3 , wherein the interferometer defines a reference arm and a measurement arm, a first difference being established between the lengths of the arms, the reference beam travelling a reference distance relative to the object, the measurement beam travelling a measurement distance relative to the object, a second difference being established between the reference distance and measurement distance, at least one arm length being established such that the first difference bears a proportional relationship to the second difference, a beat frequency being received by the receiver when the differences are substantially matched.
 6. The apparatus of claim 5 , wherein the interferometer includes at least one translationally movable path delay unit to selectively establish the at least one arm length.
 7. The apparatus of claim 1 , wherein the interferometer defines a reference arm, a measurement arm, a light entrance path, and a light exit path, the frequency shifter being positioned in one of the arms, the interferometer further including: a polarizer in the light entrance path; and a quarter wave plate positioned in the light exit path.
 8. The apparatus of claim 1 , wherein the interferometer defines a reference arm, a measurement arm, a light entrance path, and a light exit path, the frequency shifter being positioned in one of the arms, the interferometer further including: at least one spatial filter disposed in at least one of the arms to improve the quality of a wavefront of light passing therethrough.
 9. The apparatus of claim 1 , wherein the interferometer defines a reference arm and a measurement arm, and the apparatus further comprises a scanning mirror positioned in the measurement arm and tiltable in at least one degree of freedom to selectively establish a direction of propagation of a light beam impinging on the scanning mirror.
 10. The apparatus of claim 9 , wherein the plural light beams include at least a reference beam and a measurement beam directed at a measurement site defined by the object, and the apparatus further comprises a coherence layer correcting element juxtaposed with the scanning mirror to alter a length of a path traversed by the measurement beam in proportion to an angle established between the measurement beam and a null direction for the measurement beam.
 11. An apparatus for matching an object path difference with an interferometer path difference and identifying the matching by heterodyne detection of a reference beam and a measurement beam, comprising: means for shifting the frequency of at least one of: the reference beam, and the measurement beam, by at least ten megaHertz, to thereby generate a beat frequency when the object path difference bears a predetermined relationship to an interferometer path difference.
 12. The apparatus of claim 11 , wherein the means for shifting is at least one non-moving frequency shifter positioned in at least one path of the light beams, and the apparatus further comprises: an interferometer through which the beams propagate; a receiver receiving the reflected beams and generating a signal representative thereof, the signal being usable for determining the distribution of the light backscattering potential in the object, the reference beam being reflected by a reference surface defined by the object and the measurement beam being reflected by a measurement site defined by the object, such that axial motion of the object relative the interferometer equally affects the reference beam and measurement beam.
 13. The apparatus of claim 12 , wherein the frequency shifter is selected from the group of frequency shifters including acousto-optic frequency shifters, and phase modulators.
 14. The apparatus of claim 12 , wherein the interferometer defines at least a reference arm and a measurement arm, and the apparatus includes at least one translationally movable mirror or retroreflector to selectively establish the length of at least one arm.
 15. The apparatus of claim 14 , wherein the interferometer defines a light entrance path and a light exit path, the frequency shifter being positioned in one of the arms, the interferometer further including: a polarizer in the light entrance path; and a half wave plate positioned in the arm of the interferometer in which the frequency shifter is positioned.
 16. The apparatus of claim 14 , further comprising: at least one dispersion compensation element disposed in at least one of the beams.
 17. The apparatus of claim 14 , further comprising a scanning mirror positioned in the measurement arm and tiltable in at least one degree of freedom to selectively establish a direction of propagation of a light beam impinging on the scanning mirror.
 18. The apparatus of claim 17 , further comprising a coherence layer correcting element juxtaposed with the scanning mirror to alter a length of a path traversed by the measurement beam in proportion to an angle established between the measurement beam and a null direction for the measurement beam.
 19. A method for generating data representative of a three-dimensional distribution of the light backscattering potential of an object defining at least one reference surface and plural measurement sites, comprising: directing a reference beam from a reference arm of an interferometer against the reference surface; directing a measurement beam from a measurement arm of the interferometer against the measurement site; shifting the frequency of at least one beam by at least ten million Hertz; combining reflections of the beams from the surfaces; and detecting the combined reflections.
 20. The method of claim 19 , further comprising the step of altering a length of a path traversed by the measurement beam in proportion to an angle established between the measurement beam and a null direction for the measurement beam. 